1. Field of the Invention
The present invention relates generally to charged particle beam apparatus, and more specifically to electron microscopy and electron diffraction methods for high spatial resolution observation of ultrafast dynamical processes on the femtosecond scale.
2. Description of the Background Art
Optical microscopes, the simplest and most common instruments used to image objects too small for the naked eye to see, uses photons with visible wavelengths for imaging. The specimen is illuminated with a broad light beam, and a magnified image of the specimen can be observed using an eye piece or camera. The maximum magnification of a light microscope can be more than 1000× with a diffraction-limited resolution limit of a few hundred nanometers. Improved spatial resolution in an optical microscope can be achieved when shorter wavelengths of light, such as the ultraviolet, are utilized for imaging.
An electron microscope is a type of microscope that uses electrons to illuminate the specimen and create a magnified image of it. The microscope has a greater resolving power than a light microscope, because it uses electrons that have wavelengths few orders of magnitude shorter than visible light, and can achieve magnifications exceeding 1,000,000×. In a typical electron microscope, an electron beam is emitted in a vacuum chamber from an electron gun equipped with a thermionic (tungsten, LaB6), thermally assisted (Schottky, ZrO2) or cold field emission cathode. The primary electron beam, which typically has an energy ranging from a few hundred eV to few hundred keV and an energy spread ranging from few tenths to few eV, is collimated by one or more condenser lenses and then focused by the final objective lens to form a spot that illuminates the specimen. Primary electrons can generate elastically scattered electrons, secondary electrons due to inelastic scattering, characteristic Auger electrons and the emission of electromagnetic radiation. Each of the generated signals can be detected by specialized detectors, amplified and displayed on a screen or captured digitally, pixel by pixel on a computer.
Scanning electron microscopes, the most widely used electron microscopes, image the sample surface by scanning it with a tightly focused beam of electrons in a raster scan pattern, pixel by pixel. Transmission electron microscopes (TEM) and low energy electron microscopes (LEEM) are projection (as opposed to scanning) electron microscopes, and thus resemble a conventional light microscope. In a TEM or LEEM, the electron gun forms a broad electron beam that is accelerated to typically a few to hundreds of keV and focused by the objective lens. A parallel flood beam then uniformly illuminates the substrate.
The observation of dynamical processes in conventional electron microscopes is limited by the frame acquisition rate, which is typically on the order of milliseconds to seconds. However, many dynamical processes at the atomic scale occur on timescales as short as tens to hundreds of femtoseconds. Pulsed laser techniques have the requisite temporal resolution, but not the spatial resolution to follow these processes. In order to resolve structural dynamics on the ultrafast timescale, pulsed electron techniques have recently been developed. In Ultrafast electron diffraction (UED) and Dynamic transmission electron microscopy (DTEM), unprecedented combined spatial and temporal resolution is obtained by illuminating a photocathode with an ultrafast, sub-picosecond pulse generated by a pulsed laser, accelerating the photoemitted electrons and illuminating a specimen with the ultrafast electron pulse to obtain time-resolved electron diffraction patterns or images. Unfortunately, electron-electron interactions broaden the pulse as they travel from the photocathode to the sample, resulting in a loss of temporal resolution when the pulse arrives at the specimen. One option is to reduce or eliminate this broadening by greatly reducing the number of electrons in the pulse. However, the lower electron count increases the data acquisition time and in most implementations requires that the sample be reproducibly pumped and probed 106 times to obtain images or diffraction patterns of sufficient quality. Another option is to increase the beam energy into the MeV range to minimize the electron interaction time and thus the Coulomb interactions. However, electrons with such high kinetic energies not only have low scattering cross-sections yielding weak contrast, they also inevitably cause radiation damage on most types of specimens. Significant efforts have been made to shorten these ultrafast electron pulses, but there is a very strong demand to further improve the temporal resolution of the probing pulse to reach deep into the femtosecond range at typical TEM electron energies (80-120 keV) and without sacrificing the total pulse charge.
One approach to reduce or eliminate this temporal broadening is to use a Radio Frequency (RF) cavity, such as the one disclosed by T. van Oudheusden et al. in the paper entitled “Electron source concept for single-shot sub-100 fs electron diffraction in the 100 keV range” and which was published in the Journal of Applied Physics vol. 102, p. 093501, in 2007. In the RF approach, the time-varying field of an RF cavity is used to compress the pulse to the 100 femtosecond level. As the longitudinally-broadened pulse enters the RF cavity, the leading higher energy electrons are decelerated, while the trailing lower energy electrons are accelerated. At the cavity exit, the velocity-position distribution is reversed: the higher energy electrons trail the lower energy electrons. The electrons are then brought to a temporal focus on the specimen at some distance from the cavity. Both the phase and amplitude of the time-varying electric field in the cavity must be carefully tuned to the incident electron pulse, limiting the resolution of the temporal focus to ˜100 femtoseconds. Although RF methods have great potential, they lack the technical simplicity of static electron optics. Furthermore, synchronization of the RF cavity with the pulsed laser can be a challenge.
Another approach to reduce or eliminate this temporal broadening is to use a magnetic 360° deflector, such as the one disclosed in U.S. Pat. No. 8,633,438, which is entitled “Ultrafast electron diffraction device” and which was granted on Jan. 21, 2014 to inventors Tokita, Hashida and Sakabe. In this approach, the pulse is compressed by a deflector utilizing the energy dispersion of dipole magnets. The dipole magnet system utilizes static magnetic fields with a net deflection of 360 degrees. Here, temporal compression to about ˜200 femtoseconds has been achieved. Although the incident and exiting electron paths are parallel, they do not coincide due to an out of-plane offset between the magnets in the first and the second half of the deflection, which makes alignment and beam set up challenging. In addition, simple magnetic dipole deflectors exhibit focusing action mainly in the plane of deflection, necessitating the application of additional optical focusing elements. Furthermore, the deflectors introduce significant and unavoidable aberrations.
Another approach to reduce or eliminate this temporal broadening is to use a electrostatic 360° deflector, such as the one disclosed in the paper by K. Grzelakowski and R. Tromp in the paper entitled “Temporal and Lateral Electron Pulse Compression by a Compact Spherical Electrostatic Capacitor” and which was published in Ultramicroscopy vol. 130, p. 36, in 2013. In this approach, the pulse is compressed by a deflector utilizing the energy dispersion of electrostatic capacitors. The electrostatic capacitor system utilizes the central-force electrostatic field of a spherical electrostatic capacitor to compress the electron pulse. It has mirror symmetry which cancels the deflector aberrations and has compact in-line construction which provides a straight line-of-sight from source to sample. However, the symmetry invoked by the electrostatic capacitor also compresses the pulse at the deflector midplane, after a 180° deflection, which is likely to result in significant Coulomb repulsion effects.
Another approach to reduce or eliminate this temporal broadening is to use an electrostatic mirror, such as the one disclosed by G. H. Kassier et al. in the paper entitled “Achromatic reflectron compressor design for bright pulses in femtosecond electron diffraction” and which was published in the Journal of Applied Physics vol. 105, p. 113111, in 2009, and such as the one disclosed by Y. Wang and N. Gedik in the paper entitled “Electron Pulse Compression With a Practical Reflectron Design for Ultrafast Electron Diffraction” and which was published in IEEE Journal of selected topics in quantum electronics, vol. 18, p. 140, in 2012. In this approach, the pulse is compressed because the higher energy electrons penetrate more deeply into the mirror, resulting in a longer beam path. For a substantial beam path differential between the high and low energy electrons, the higher energy electrons will trail the lower energy electrons, resulting in pulse compression downstream. However, these mirror compressors share a common feature: the pulse enters the mirror at a relatively large entrance angle in order to separate the incoming and outgoing pulse. The skewed entry complicates the overall column alignment, as there is no direct line of sight from the electron source to the specimen. In addition, the large entrance angle introduces significant aberrations and a significant tilt of the pulse front, which complicates the experimental setup.
There is significant demand in biological and medical research as well materials science and semiconductor processing for imaging of specimens at high temporal and spatial resolution. An improved pulse compressor and methods for providing temporal compression that overcome the disadvantages of the above-mentioned approaches are desirable.